Street lighting systems may be operated by manually operated switches, but more frequently they are operated under the control of a photoelectric cell or timer or a programmed combination of both a timer and a photoelectric cell. In conventional such systems, steady-state power is applied to the load at full rated voltage.
Experience shows that street lights may operate at power levels substantially below rated power levels without causing a perceptible decline in the illumination provided. While during peak night-time traffic hours it may be desirable to operate the street lamps at full rated voltage, nevertheless in the middle night-time hours when traffic intensity is low and many people are asleep, municipalities for economic reasons may choose to operate street lamps at reduced power levels. Satisfactory control circuits to permit reduced power level operation (sufficient to provide satisfactory off-peak illumination) to occur automatically, subject to various overrides to deal with particular circumstances, are not readily available.